Sheet discharging device and image forming apparatus including the sheet discharging device

ABSTRACT

In a sheet discharging unit including a pair of sheet discharge rollers which discharges a sheet P, the outer diameter of a sheet discharge drive roller included in the pair of sheet discharge rollers is set so that the outer diameter of a straight portion which is a central portion in the width direction perpendicular to the discharging direction of the sheet P is larger than the outer diameters of both end portions in the width direction. Further, in the sheet discharging unit, the pair of sheet discharge rollers respectively include metal shafts extending in the width direction and elastic layers respectively wound around the metal shafts and continuously formed to be longer in the width direction than the width of a sheet P with a passable maximum size. Further, at least one elastic layer of the pair of sheet discharge rollers is formed of a foamed material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet discharging device of an image forming apparatus, which uses an electrostatic recording system and an electrophotographic recording system, and an image forming apparatus including the sheet discharging device.

2. Description of the Related Art

Recently, an image forming apparatus has reduced in size and realized high performance, and an intermediate transfer type full-color image forming apparatus having the constitution disclosed in U.S. Pat. No. 6,970,665 has been developed. The invention disclosed in U.S. Pat. No. 6,970,665 is described with reference to FIG. 5 of the present application.

As illustrated in FIG. 5, an image forming apparatus 200 includes a plurality of image forming portions 10. The image forming portions 10 form a latent image on photosensitive drums 11, which are image bearing members, with the use of light, magnetism, and charge to elicit the latent image with a developer such as a toner, and, thus, to obtain a visible image. The image forming apparatus 200 further includes an intermediate transfer unit 30 provided above the image forming portions 10 and a secondary transfer roller 36. The visible images from the image forming portions 10 are sequentially transferred to the intermediate transfer unit 30, and the intermediate transfer unit 30 forms an image of a plurality of colors. The secondary transfer roller 36 transfers the image of a plurality of colors on the intermediate transfer unit 30 onto a sheet P. The image forming apparatus 200 further includes a fixing unit 40 for fixing the image of a plurality of colors, transferred onto the sheet P, onto the sheet P and a sheet discharging portion 250 for discharging the sheet P fixed with the image to the outside of the image forming apparatus 200. Furthermore, a sheet conveyance unit 20 for conveying the sheet P to a secondary transfer portion Te, and a manual paper feed tray 71 and sheet cassette 21 for feeding the sheet P to the sheet conveyance unit 20 are disposed below the secondary transfer roller 36 of the image forming apparatus 200.

The intermediate transfer unit 30 includes a rotating endless belt-like intermediate transfer belt 31 stretched between a plurality of rollers, and the intermediate transfer belt 31 is disposed to approach the image forming portions 10. In the intermediate transfer unit 30, a primary transfer charging devices 35 are arranged at a position facing the image forming portions 10 so that the intermediate transfer unit 30 is held between the primary transfer charging devices 35 and the image forming portions 10. The primary transfer charging devices 35 primarily transfers the visible images, formed on the photosensitive drums 11 by an exposure device 6, onto the intermediate transfer-unit 30. The visible images primarily transferred from the photosensitive drums 11 are superposed on the intermediate transfer unit 30. The intermediate transfer unit 30 rotates to thereby convey the visible image to the position of the secondary transfer portion Te where the visible image is secondarily transferred onto the sheet P.

When the visible image on the intermediate transfer unit 30 arrives at the position of the secondary transfer portion Te, the visible image is secondarily transferred onto the sheet P, selected and conveyed from the manual paper feed tray 71 or a sheet cassette 21, by the secondary transfer roller 36 and is further fixed by the fixing unit 40. The sheet P fixed with the image is discharged outside the image forming apparatus 200 by the sheet discharging portion 250, whereby a full-color image can be obtained. A sheet discharge tray 60 is disposed on the downstream side in the sheet conveyance direction of the sheet discharging portion 250. The sheet discharge tray 60 can receive therein the sheet P discharged by the sheet discharging portion 250.

For the sheet discharging portion 250, as disclosed in Japanese Patent Application Laid-Open No. 2006-117365, a plurality of rollers 502 with a width smaller than the width of a plurality of concavo-convex rollers 501 are arranged in a row in the width direction perpendicular to the discharging direction of the sheet P. The invention disclosed in Japanese Patent Application Laid-Open No. 2006-117365 is described with reference to FIG. 6.

As illustrated in FIG. 6, the sheet discharging portion 250 includes the plurality of concavo-convex rollers 501. According to the sheet discharging portion 250, the sheet P is stiffened, whereby while curl of the sheet P is removed, the sheet P can be discharged while always stabilizing the posture of the sheet P at the time of discharge. The lead end of the sheet P discharged does not push out the sheets P previously stacked in the sheet discharge tray 60, but the sheet P is discharged in a state that the sheets P are neatly arranged in the sheet discharge tray 60.

Meanwhile, for the sheet discharging portion 250, instead of the concavo-convex rollers 501 of Japanese Patent Application Laid-Open No. 2006-117365 and FIG. 6, USPub2007/0201892 proposes to use a pair of sheet discharge rollers in which a rubber layer wound around a roller shaft has a length larger than the width of the sheet P. The invention disclosed in the USPub2007/0201892 is described with reference to FIG. 7.

As illustrated in FIG. 7, according to a sheet discharging portion 350, the total width of elastic layers of a sheet discharge drive roller 503 and a sheet discharge driven roller 504 is evenly in contact with the sheet P, and therefore, heat of the sheet P is uniformly conducted away, whereby uneven brightness of an image can be prevented. Further, since the edge portions of the rollers are not in contact with the sheet P, it is possible to prevent the traces of the rollers from being left on the sheet P.

However, when the concavo-convex rollers 501 illustrated in FIG. 6 are used, the surfaces of the rollers are nonuniformly in contact with the sheet P. In this case, for the sheet P having passed through the fixing unit 40, since the portions of the sheet P which are in contact with the roller surfaces and the noncontact portions are different in the amount of heat of the sheet P conducted by the roller surfaces, the cooling conditions of a toner image fixed onto the sheet P results in difference. Therefore, the portions which are in contact with the roller surfaces and the noncontact portions cause the uneven brightness of the image on the sheet P. In particular, when the sheet P with high stiffness, like a heavy paper, is used, the contact surface between the sheet P and the roller surface and the noncontact surface clearly appear, and therefore, the-uneven brightness more notably occurs.

The plurality of rollers 502 are aligned in the width direction, and the sheet P is stiffened by each edge portion of the rollers 502. Therefore, the traces of the rollers are left on the sheet P, softened by the heat when fixed, by the corner portions of the rollers 502.

In addition, the sheet discharging portion 350 of U.S. Pat. No. 6,970,665 and FIG. 7 cause the following problem.

As illustrated in FIG. 7, the sheet discharging portion 350 is constituted of the pair of sheet discharge rollers including the sheet discharge drive roller 503 and the sheet discharge driven roller 504 which have a uniform diameter and are through rollers continued in the width direction of the sheet P. The sheet discharge tray 60 is disposed on the downstream side of the sheet discharging unit 50.

As illustrated in FIG. 7, a pair of sheet discharge roller 505 is a straight pair of roller having a uniform outer diameter over the whole length in the width direction and does not have a function for stiffening the sheet P. Thus, a sheet P1 is discharged on the sheet discharge tray 60 so that a lead end portion Pa is discharged substantially linearly with respect to the pair of sheet discharge roller 505 and, at the same time, so that the sheet P1 is discharged in a curled fashion in the sheet discharging direction. At this time, the sheet P1 is discharged in the direction of the arrow A with respect to the sheet P0, which has been discharged and stacked on the sheet discharge tray 60, so that the entire region of the lead end portion Pa (a dotted line region in FIG. 7) is pressed against the sheet P0. Therefore, the sliding resistance between the sheet P1 and the sheet P0 increases, and the sheet P0 may be pushed out in the direction of the arrow B. In addition, the lead end portion Pa of the sheet P1 may be caught on the sheet P0 to be curled on the sheet discharge tray 60 due to a surface property of the sheet P or a minimal burr, which is generated at the end of the sheet P when the sheet P is cut. Thus, the sheet to be discharged subsequently cannot be normally discharged, and the sheet P1 is highly likely to be pushed outside the sheet discharge tray 60, or sheet jamming easily occurs, resulting in extreme deterioration of sheet dischargeability.

In addition, the amount of heat of the sheet P1 conducted by the pair of sheet discharge roller 505 is different between the front surface and the rear surface of the sheet P1 due to the difference in material of the pair of sheet discharge roller 505, whereby shrinkage of the sheet P1 in the cooling of the sheet P1 is different between the front surface and the rear surface, and thus, the sheet may be likely to be curled.

The present invention has been made in view of the above problems, and the present invention provides a sheet discharging device, which can increase the rigidity of a sheet in a sheet discharging direction, stabilize the posture of a discharged sheet, and realize the improvement of loading performance of the discharged sheet.

SUMMARY OF THE INVENTION

The present invention provides a sheet discharging device comprising a sheet stacking portion on which sheets are staked, and a pair of sheet discharge rollers which nips and discharges a sheet, on which a toner image is fixed by heat, onto the sheet stacking portion in a convex shape in a width direction perpendicular to the discharging direction of the pair of sheet discharge rollers.

Each of the pair of sheet discharge rollers includes a rotating shaft extending in the width direction and an elastic layer is provided on the rotating shaft and continuously formed to be longer in the width direction than the width of a sheet with a passable maximum size, and at least one elastic layer of the pair of sheet discharge rollers is formed of a foamed material.

According to the present invention, a pair of sheet discharge rollers has an elastic layer. The sheet is nipped and discharged by the pair of sheet discharge rollers in a convex shape (U-shaped curve) in a width direction, whereby the rigidity of the sheet in the sheet discharging direction increases, the posture of the discharged sheet is stabilized, and the loading performance of the discharged sheet is improved. The sheet and the roller surface are evenly in contact with each other by the elastic layer, at least one elastic layer is formed of a foamed material, of the pair of sheet discharge rollers, and thus, the uneven brightness can be prevented when an image is fixed onto the sheet. Further, since the corner portions of the roller are not in contact with the sheet, the traces of the rollers are prevented from being left on the sheet.

Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention;

FIG. 2 is an enlarged side elevational view illustrating a configuration of a sheet discharging unit as viewed from the downstream side in a conveyance direction of a sheet;

FIG. 3 is a schematic perspective view illustrating configurations of the sheet discharging unit and a sheet discharge tray;

FIG. 4 is an enlarged side elevational view illustrating a configuration of a sheet discharging unit, used in an image forming apparatus according to a second embodiment of the present invention, as viewed from the downstream side in a conveyance direction of a sheet;

FIG. 5 is a cross-sectional view illustrating a configuration of the prior image forming apparatus;

FIG. 6 is an enlarged side elevational view illustrating a configuration of the prior sheet discharging unit; and

FIG. 7 is a schematic perspective view illustrating configurations of the prior sheet discharging unit and the prior sheet discharge tray.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

FIG. 1 is a cross-sectional view illustrating a configuration of an image forming apparatus 100 according to the first embodiment of the present invention. The image forming apparatus 100 is a color image forming apparatus using an electrophotographic image forming process.

As illustrated in FIG. 1, the image forming apparatus 100 includes an image forming portion 1 a for forming a yellow image and an image forming portion 1 b for forming a magenta image. The image forming apparatus 100 further includes an image forming portion 1 c for forming a cyan image and an image forming portion 1 d for forming a black image. The image forming apparatus 100 includes those four image forming portions (image forming unit). Those four image forming portions 1 a, 1 b, 1 c, and 1 d are arranged in a row at a constant interval. The image forming portions 1 a, 1 b, 1 c, and 1 d respectively include drum-type electrophotographic photoreceptor (hereinafter referred to as photosensitive drums) 2 a, 2 b, 2 c, and 2 d serving as image bearing members forming a visible image. Around the respective photosensitive drums 2 a, 2 b, 2 c, and 2 d, there are provided chargers 3 a, 3 b, 3 c, and 3 d, developing devices 4 a, 4 b, 4 c, and 4 d, drum cleaning devices 5 a, 5 b, 5 c, and 5 d. An exposure device 6 is provided below the image forming portions 1 a, 1 b, 1 c, and 1 d. The developing devices 4 a, 4 b, 4 c, and 4 d respectively contain yellow, magenta, cyan, and black toners.

Each of the photosensitive drums 2 a, 2 b, 2 c, and 2 d is a negatively charged OPC photoreceptor having a photoconductive layer on a drum base body made of aluminum, and is driven and rotated by a drive (not illustrated) in the direction of the arrow (the clockwise direction) at a predetermined process speed. The chargers 3 a, 3 b, 3 c, and 3 d serving as charging portions uniformly charge the surface of the photosensitive drums 2 a, 2 b, 2 c, and 2 d to a predetermined negative electrode potential by a charging bias applied from a charging bias supply (not illustrated). The developing devices 4 a, 4 b, 4 c, and 4 d cause the toners of the respective colors to adhere to the electrostatic latent images formed on the photosensitive drums 2 a, 2 b, 2 c, and 2 d to develop (visualize) the electrostatic latent images as toner images. The developing method performed by the developing devices 4 a, 4 b, 4 c, and 4 d includes a two-component contact developing method. In this developing method, a developer containing a magnetic carrier mixed with toner particles is conveyed by a magnetic force, and image development is performed in a contact state with the photosensitive drums 2 a, 2 b, 2 c, and 2 d.

Primary transfer rollers 34 a, 34 b, 34 c, and 34 d serving as transfer portions are constituted of an elastic member and abutted against the photosensitive drums 2 a, 2 b, 2 c, and 2 d through an endless belt-like intermediate transfer belt 31 in each transfer nip portion. In this embodiment, the primary transfer rollers 34 a, 34 b, 34 c, and 34 d are used as the transfer portions; however, there may be used a transfer blade which is subjected to high pressure when transferring a toner image to a transfer material and is abutted against the intermediate transfer belt 31.

The drum cleaning devices 5 a, 5 b, 5 c, and 5 d remove and collect a transfer residual toner remaining on the respective surfaces of the photosensitive drums 2 a, 2 b, 2 c, and 2 d.

In the exposure device 6, a laser beam modulated in response to a time series electric digital pixel signal of image information is output from a laser outputting portion (not illustrated) . The exposure device 6 exposes the surfaces of the photosensitive drums 2 a, 2 b, 2 c, and 2 d through, for example, a polygon mirror (not illustrated) rotating at high speed. According to this constitution, electrostatic latent images of the respective colors corresponding to the image information are formed on the surfaces of the photosensitive drums 2 a, 2 b, 2 c, and 2 d charged by the chargers 3 a, 3 b, 3 c, and 3 d.

The sheet conveyance unit 20 includes a sheet cassette 21, a cassette conveyance roller 22, which is a conveying portion, registration rollers 23 a and 23 b, which are conveying portions, a manual paper feed tray conveyance roller 24, and a manual paper feed tray 71. The sheet conveyance unit 20 selects the sheet P in the sheet cassette 21 or on the manual paper feed tray 71 to convey the selected sheet P, and conveys the sheet P to the secondary transfer portion Te which is a transfer portion. The secondary transfer portion Te transfers the visible image, formed by the image forming portions 1 a, 1 b, 1 c, and 1 d, to the sheet P.

In the intermediate transfer unit 30, the intermediate transfer belt 31 is stretched between a drive roller 32 and a tension roller 33, and the intermediate transfer belt 31 is driven by the drive roller 32 to be rotated (moved) in the arrow direction (the counterclockwise direction). The intermediate transfer belt 31 is formed of a dielectric resin such as a polycarbonate, a polyethylene terephthalate resin film, or a polyvinylidene fluoride resin film. Further, an intermediate transfer belt cleaning device 80 is provided in a position facing the tension roller 33 through the intermediate transfer belt 31. The intermediate transfer belt cleaning device 80 includes a cleaning blade 81 formed of an elastic body abutted against the intermediate transfer belt 31 at a predetermined pressure and a conveying screw 82 which conveys residual toner removed from the intermediate transfer belt 31 by the cleaning blade 81. The residual toner is conveyed to a toner collection vessel (not illustrated) by the conveying screw 82.

The fixing unit 40 is provided on the downstream side of the secondary transfer portion Te. The fixing unit 40 is a fixing portion having a fixing roller 42 including a heat source and a pressure roller 41. The fixing unit 40 fixes the visible image on the sheet P by heat, transferred at the secondary transfer portion Te, to the sheet P. Further, a sheet discharging unit 50 which is a sheet discharging device is provided on the downstream side in the conveyance direction of the sheet P. The sheet discharge tray 60 is disposed on the further downstream side in the sheet passing direction of the sheet discharging unit 50 and above the intermediate transfer unit 30. The sheet P discharged by the sheet discharging unit 50 is stacked on the sheet discharge tray 60 serving as a sheet stacking portion.

Next, an image forming operation performed by the image forming apparatus 100 is described. When an image formation starting signal is given, the photosensitive drums 2 a, 2 b, 2 c, and 2 d of the image forming portions 1 a, 1 b, 1 c, and 1 d, which are driven and rotated at a predetermined process speed, are uniformly negatively charged by the chargers 3 a, 3 b, 3 c, and 3 d. Then, the exposure device 6 converts an image signal of an output image into an optical signal by means of a laser outputting portion (not illustrated), and the laser beam which is the optical signal obtained by conversion scans and exposes each surface of the charged photosensitive drums 2 a, 2 b, 2 c, and 2 d to form the electrostatic latent image.

Subsequently, the electrostatic latent image formed on the photosensitive drum 2 a is first adhered with yellow toner by the developing device 4 a to which a developing bias having the same polarity as the charged polarity (negative electrode polarity) of the photosensitive drum 2 a is applied, and the electrostatic latent image is developed as the toner image. Then, the yellow toner image is transferred onto the intermediate transfer belt 31 at a primary transfer portion Ta by the primary transfer roller 34 a to which a transfer bias (opposite-polarity (straight polarity) against the toner) is applied.

The intermediate transfer belt 31 transferred with the yellow toner image is moved to the image forming portion 1 b by the drive roller 32. Likewise, a magenta toner image, formed on the photosensitive drum 2 b by the above same method, is superposed on the yellow toner image on the sheet P, and then transferred at a primary transfer portion Tb constituted of the image forming portion 1 b and the primary transfer roller 34 b. Hereinafter, likewise, a cyan toner image and a black toner image, respectively formed on the photosensitive drums 2 c and 2 d of the image forming portions 1 c and 1 d, are sequentially superposed on the yellow and magenta toner images superposed and transferred onto the intermediate transfer belt 31, at primary transfer portions Tc and Td. According to this constitution, a full color toner image is formed on the intermediate transfer belt 31.

The sheet P conveyed from the sheet cassette 21 or the manual paper feed tray 71 is conveyed to the secondary transfer portion Te by the registration rollers 23 a and 23 b according to the timing when the lead end of the toner image on the intermediate transfer belt 31 is moved to the secondary transfer portion Te. The full color toner image is then transferred onto the sheet p, conveyed to the secondary transfer portion Te, by the secondary transfer roller 36 to which the transfer bias (opposite-polarity (straight polarity) against the toner) is applied.

The sheet P formed with the full color toner image is conveyed to the fixing unit 40 serving as a fixing portion which is an image heat fixing device. After the full color toner image is heated and pressurized by a fixing nip between the fixing roller 42 and the pressure roller 41 to be thermally fixed onto the surface of the sheet P, the sheet P is discharged into the sheet discharge tray 60, provided outside the apparatus, by the sheet discharging unit 50, whereby a series of the image forming operation is terminated.

FIG. 2 is an enlarged side elevational view illustrating a configuration of the sheet discharging unit 50 as viewed from the downstream side in the conveyance direction of the sheet P. As illustrated in FIG. 2, the sheet discharging unit 50 which is a sheet discharging device includes a pair of sheet discharge rollers including a sheet discharge drive roller 51 and a sheet discharge driven roller 52. A sheet discharge drive roller 51 is corresponds to a one sheet discharge roller. A sheet discharge driven roller 52 is corresponds to an other sheet discharge roller. The sheet discharge drive roller 51 transmits a driving force of a drive source (not illustrated). The sheet discharge driven roller 52 faces the sheet discharge drive roller 51 and applies pressure on the sheet discharge drive roller 51 to be biased. The sheet discharging unit 50 discharges the sheet P, which has passed through the fixing unit 40 and, on which a toner image is fixed by heat, outside an image forming apparatus body 100 a by the rotation of the sheet discharge drive roller 51 and the sheet discharge driven roller 52.

The sheet discharge drive roller 51 includes a metal shaft 51 b, which is a rotating shaft, and an elastic layer 51 a is provided on the metal shaft 51 b. The elastic layer 51 a is formed of a resin material such as a foamed rubber material or a foamed elastomer, which is a foamed material, and, for example, urethane, NBR, and EPDM are used therein. In this case, in order to reliably grip and convey the sheet, the hardness of the rubber material is preferably 20 to 60 (when the hardness is measured by Asker C hardness meter). The sheet discharge drive roller 51 is connected to a drive source (not illustrated), and is rotatably constituted as a drive roller. The elastic layer 51 a is continuously formed in the direction along the metal shaft 51 b.

Meanwhile, the sheet discharge driven roller 52 includes a metal shaft 52 b, which is a rotating shaft, and an elastic layer 52 a is provided on the metal shaft 52 b. Like the sheet discharge drive roller 51, the elastic layer 52 a of the sheet discharge driven roller 52 is formed of a resin material such as a foamed rubber material or a foamed elastomer, which is a foamed material, and, for example, urethane, NBR, and EPDM are used therein. At least one elastic layer of the sheet discharge drive roller 51 and the sheet discharge driven roller 52 maybe formed of a foamed material, the total amount of heat conducted from the sheet P is reduced and the sheet P is stacked on the sheet discharge tray 60 with flexibility, whereby curling of the discharged sheet P is reduced. Further, it is preferable that the sheet discharge driven roller 52 is formed of the same material as in the sheet discharge drive roller 51 or a material having the same heat conductivity as that of the sheet discharge drive roller 51 to reduce curling of the discharged sheet P. Furthermore, as in the sheet discharge drive roller 51, in order to reliably grip and convey the sheet, the hardness of the rubber material of the sheet discharge driven roller 52 is preferably 20 to 60 (when the hardness is measured by Asker C hardness meter). The elastic layer 52 a is continuously formed in the direction along the metal shaft 52 b. The sheet discharge driven roller 52 includes pressure springs 54 disposed at the both ends of the metal shaft 52 b through a bearing 53, and the pressure springs 54 apply a predetermined pressure to the sheet discharge drive roller 51. The pressure of the pressure spring 54 applied to the sheet discharge drive roller 51 is preferably set to about 0.1 to 1 kg so that the conveying performance for the sheet P can be ensured.

The elastic layer 51 a of the sheet discharge drive roller 51 has a substantially symmetrical shape with respect to a center line a of the center in the width direction of the sheet P. The elastic layer 51 a is constituted of a straight portion 61, which is a cylindrical portion without a change of the outer diameter, and taper portion 62 having a tapered shape with a taper amount d1 from the both end portions of the straight portion 61 b to the both end portions of the elastic layer 51 a. A region of the center line a is at least referred to as a central portion of the sheet discharge drive roller 51. A region of the straight portion 61 also corresponds to the central portion of the sheet discharge drive roller 51. In the elastic layer 52 a of the sheet discharge drive roller 51, the outer diameter D1 of the central portion is set to be larger than the outer diameter D2 of the both end portions 62 a which are the both end portions in the width direction.

When the length L in the axial direction of the straight portion 61 is too long, the effect of the difference between the outer diameter D1 of the straight portion 61 of the elastic layer 51 a and the outer diameters D2 of the both end portions 62 a: D1-D2 may be reduced. On the other hand, if the length L is too short, a balance of holding the sheet P is likely to be disrupted in the axial direction, thereby leading to skew feeding of the sheet P, or normal discharging may not be able to be performed. Thus, the length L in the axial direction of the straight portion 61 is preferably larger than 0 but equal to or less than ⅓ with respect to the length L1 (see, FIG. 3) in the width direction of the sheet P with a passable maximum size. Namely, it is preferable that 0<L≦⅓L1 is established.

FIG. 3 is a schematic perspective view illustrating the configurations of the sheet discharging unit 50 and the sheet discharge tray 60. As illustrated in FIG. 3, the sheet discharge tray 60 is disposed on the downstream side in the passing direction of the sheet P of the sheet discharging unit 50. In this case, when the sheet P is discharged, the sheet P is conveyed on a center basis regardless of the size of the sheet P.

According to the sheet discharging unit 50 of the present embodiment, the sheet discharge drive roller 51 has the difference between the outer diameter D1 of the straight portion 61 and the outer diameters D2 of the both end portions 62 a: D1-D2. Therefore, the circumferential speed of the sheet discharge drive roller 51 is different between the straight portion 61 and the both end portions 62 a. While the circumferential speed of the straight portion 61 is high, each circumferential speed of the both end portions 62 a is lower than that of the straight portion 61. Thus, the sheet discharge speed of the discharged sheet P is different between the central portion Pb and the both end portions Pc. As illustrated by the arrow C in FIG. 3, the central portion Pb of the lead end portion of the sheet P is discharged at a higher speed than the both end portions Pc of the lead end portion of the sheet P, whereby the central portion Pb of the lead end portion of the sheet P is discharged earlier than the both end portions Pc of the lead end portion of the sheet P. Since the both end portions Pc of the lead end portion of the sheet P are pulled by the central portion Pb of the lead end portion of the sheet P, the both end portions Pc is discharged in a slightly lifted state to follow the shape of the taper portions 62. Thus, when the sheet P1 is discharged, the lead end portion of the sheet P1 can be discharged while only the central portion Pb of the lead end portion of the sheet P1 is in contact with the sheet P0 already discharged on the sheet discharge tray 60.

As described above, according to the image forming apparatus 100 of the first embodiment, the outer diameters of the elastic layer 51 a of the sheet discharge drive roller 51 are formed to be substantially symmetrical to each other with the central portion in the width direction of the sheet P as the center, and the outer diameter D1 of the straight portion 61 is set to be larger than the outer diameters D2 of the both end portions 62 a. The elastic layer 51 a has the smooth taper shapes from the straight portion 61 to the both end portions 62 a. Thus, the discharge speed of the sheet P is set to be high in the straight portion 61, but the discharge speeds in the both end portions 62 a are set to be lower than the discharge speed in the straight portion 61. Therefore, the sheet P is nipped and discharged by the pair of sheet discharge rollers in a convex shape (U-shaped curve) that the central portion Pb is downward, and the both end portions Pc of the lead end portion of the sheet P are slightly lifted upward in the width direction perpendicular to the discharging direction of the pair of sheet discharge rollers. According to this constitution, the rigidity of the sheet in the sheet discharging direction can be increased, and the contact area between the lead end of the discharged sheet P1 and the already discharged sheet P0 can be reduced in size as much as possible. Consequently, it is possible to prevent such a trouble that the discharged sheet P1 is caught by the already discharged sheet P0 to push out the sheet P0. Further, the sheet jamming is prevented. Furthermore, the dischargeability and the loading performance of the discharged sheet P can be enhanced. In addition, it is possible to prevent, as much as possible, such a phenomenon that the lead end portion of the discharged sheet P is caught to be curled due to the surface property of the sheet P, which has been already discharged on the sheet discharge tray 60, or a minimal burr, which is generated at the end of the sheet P when the sheet P is cut. The sheet P may be nipped and discharged by the pair of sheet discharge rollers in a reverse convex shape (inverted U-shaped curve) to increase the rigidity of the sheet in the sheet discharging direction.

The sheet discharge drive roller 51 and the sheet discharge driven roller 52 respectively have the elastic layers on the outer circumferential sides, whereby when the sheet P is passed between the sheet discharge drive roller 51 and the sheet discharge driven roller 52 while being pressed therebetween, the straight portion 61 of the sheet discharge drive roller 51 can be elastically depressed according to the thickness of the sheet P. The elastic layer 51 a can be evenly in contact with the sheet P. According to this constitution, there is no difference in the amount of heat of the toner image on the sheet P conducted by the surfaces of the sheet discharge drive roller 51 and the sheet discharge driven roller 52, and thus, the occurrence of the uneven brightness of an image can be prevented.

The elastic layer 51 a is continued in the direction along the metal shaft 51 b. The edge portion of the elastic layer 51 a is not pressed against a softened sheet, and thus, the traces of the rollers are not left on the sheet, whereby the image with fine printing image quality is output.

The sheet discharge drive roller 51 and the sheet discharge driven roller 52 respectively have the foam elastic layers 51 a and 52 a, whereby each heat capacity of the sheet discharge drive roller 51 and the sheet discharge driven roller 52 is set to be reduced, and, thus, the total amount of heat conducted from the sheet P is reduced. The discharged sheet P is not cooled more than necessary, so the discharged sheet P does not become hard. After the sheet P is stacked on the sheet discharge tray 60 with flexibility, the sheet P is stacked on a flat stacking surface of the sheet discharge tray 60 and is cooled, whereby curling of the discharged sheet P is reduced. Furthermore, the amount of heat conducted from the sheet P is uniformed on the front and rear sides of the sheet P, whereby corrugation or curling of the discharged sheet P is reduced.

The sheet discharge drive roller 51 in which the outer diameter of the straight portion 61 is larger than the outer diameters of the both end portions 62 a is disposed above the sheet discharge driven roller 52. Thus, the sheet P is discharged by being pressed from the central position in the width direction of the upper surface of the sheet P. Consequently, when the sheet P passes between the sheet discharge drive roller 51 and the sheet discharge driven roller 52, the sheet P is likely to be discharged while being curled into a reversed U shape. The central position in the width direction of the discharged sheet P1 is hardly caught by the already discharged sheet P0. On the other hand, if the straight portion 61 and the taper portions 62 are formed in the sheet discharge driven roller 52, the sheet P may be inconveniently discharged while being curled into a U shape.

Since the length of the straight portion 61 is equal to or less than ⅓ of the length L1 in the width direction of the sheet P with a passable maximum size, the gripping force for the sheet P of the sheet discharge drive roller 51 and the sheet discharge driven roller 52 is suitably maintained. When the sheet P with high stiffness, like a heavy paper, is used, the entire region in the width direction of the sheet P can be reliably nipped. Therefore, the uneven brightness can be prevented. If the length of the straight portion 61 is more than ⅓ of the maximum width of the passable sheet P, there hardly occurs a difference in the discharge speed between the straight portion 61 in the width direction of the sheet P and the both end portions 62 a. When the straight portion 61 is not provided, a balance of holding the sheet P is likely to be disrupted in the axial direction, thereby leading to skew feeding of the sheet P, or normal discharging may not be able to be performed.

As described above, according to the first embodiment of the present invention, based on a simple constitution, the image forming apparatus 100 can be reduced in size and cost while maintaining high quality printed image and enhancing the dischargeability of the sheet P.

It goes without saying that this embodiment is not limited to a developing system and a transferring system used in the image forming apparatus described in this embodiment, but widely effective in a discharge portion of an image forming apparatus.

Second Embodiment

FIG. 4 is an enlarged side elevational view illustrating a configuration of a sheet discharging unit 150, used in the image forming apparatus according to the second embodiment of the present invention, as viewed from the downstream side in the conveyance direction of the sheet P. The sheet discharging unit 150 is different from the sheet discharging unit 50 of the first embodiment in the use of a sheet discharge drive roller 151 instead of the sheet discharge drive roller 51. In the sheet discharging unit 150, the same components and effects as those in the sheet discharging unit 50 are denoted by the same reference numerals, and the description is suitably omitted. Also in the second embodiment, the image forming apparatus similar to that of the first embodiment can be used, and thus the description of the image forming apparatus is omitted.

As illustrated in FIG. 4, the sheet discharging unit 150 is constituted of a sheet discharge drive roller 151 and a sheet discharge driven roller 52. The sheet discharge drive roller 151 is constituted of a metal shaft 51 b, which is a rotating shaft, and an elastic layer 151 a wound around the metal shaft 51 b. The elastic layer 151 a has a crown shape with a maximum outer diameter D1 in the central portion in the width direction of the sheet P. The elastic layer 151 a is formed of a foam rubber material, which is a foamed material, and, for example, urethane, NBR, and EPDM are used therein. In this case, in order to reliably grip and convey the sheet P, the hardness of the rubber material is preferably 20 to 60 (when the hardness is measured by Asker Chardness meter). The sheet discharge drive roller 151 is connected to a drive source (not illustrated), and is rotatably constituted as a drive roller.

Meanwhile, the sheet discharge driven roller 52 is constituted of a metal shaft 52 b and an elastic layer 52 a wound around the metal shaft 52 b. As in the sheet discharge drive roller 151, the elastic layer 52 a of the sheet discharge driven roller 52 is formed of a foam rubber material, and, for example, urethane, NBR, and EPDM are used therein. Further, it is preferable that the sheet discharge driven roller 52 is formed of the same material as in the sheet discharge drive roller 151 or a material having the same heat conductivity as that of the sheet discharge drive roller 151. Further, as in the sheet discharge drive roller 151, the hardness of the rubber material of the sheet discharge driven roller 52 is preferably 20 to 60 (when the hardness is measured by Asker C hardness meter). The sheet discharge driven roller 52 includes pressure springs 54 disposed at the both ends of the metal shaft 52 b through a bearing 53, and the pressure springs 54 apply a predetermined pressure to the sheet discharge drive roller 151. The pressure of the pressure spring 54 applied to the sheet discharge drive roller 151 is preferably set to about 0.1 to 1 kg so that the conveying performance for the sheet P can be ensured.

The elastic layer 151 a of the sheet discharge drive roller 151 has a substantially symmetrical shape with respect to the center line a of the center in the width direction of the sheet P, and the outer shape is a smooth crown shape with a crown amount d2.

The image forming apparatus of the second embodiment can obtain an effect similar to the case of the first embodiment. The elastic layer 151 a of the sheet discharge drive roller 151 has a crown shape with the maximum outer diameter D1 in the central portion in the width direction of the sheet P. Therefore, as an additional effect, the elastic layer 151 a of the sheet discharge drive roller 151 can be smoothly deformed in accordance with the shape of the sheet P in the width direction of the sheet P.

Note that the above constitution can be changed as follows. Among the elastic layers 51 a, 151 a, and 52 a of the sheet discharge drive rollers 51 and 151 and the sheet discharge driven roller 52 of the first and second embodiments, a tube-like resin film may be wound around the outer circumference of at least one of the elastic layers 51 a, 151 a, and 52 a disposed on the printed surface side of the sheet. According to this constitution, even if the toner is not completely fusion bonded to the sheet P in the fixing unit 40, the toner is prevented from being removed by the elastic layers 51 a, 151 a, and 52 a.

The elastic layers 51 a and 151 a of the sheet discharge drive rollers 51 and 151 of the first and second embodiments may be set to have a hardness higher than that of the elastic layer 52 a of the sheet discharge driven roller 52. According to this constitution, each hardness of the elastic layers 51 a and 151 a of the sheet discharge drive rollers 51 and 151 is higher than the hardness of the elastic layer 52 a of the sheet discharge driven roller 52. Thus, it is possible to prevent such a phenomenon that the elastic layers 51 a and 151 a of the sheet discharge drive rollers 51 and 151 are gridded by the sheet discharge driven roller 52. Consequently, the difference between the outer diameter D1 of the straight portion 61 and the outer diameters D2 of the both end portions 62 a: D1-D2, in the sheet discharge drive rollers 51 and 151 is maintained for a long period of time.

Further, in the first and second embodiments, the sheet discharging device according to the present invention is incorporated in the image forming apparatus; however, the sheet discharging device maybe a finisher which can be connected from the outside of the image forming apparatus.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2008-201811, filed Aug. 5, 2008, No. 2009-162516, filed Jul. 9, 2009 which are hereby incorporated by reference herein in their entirety. 

1. A sheet discharging device comprising: a sheet stacking portion on which sheets are staked; and a pair of sheet discharge rollers which discharges a sheet, on which a toner image is fixed by heat, onto the sheet stacking portion in a U-shaped curve in a width direction perpendicular to the discharging direction of the pair of sheet discharge rollers, wherein each of the pair of sheet discharge rollers includes a rotating shaft extending in the width direction and an elastic layer is provided on the rotating shaft and continuously formed to be longer in the width direction than the width of a sheet with a passable maximum size, and at least one elastic layer of the pair of sheet discharge rollers is formed of a foamed material.
 2. The sheet discharging device as claimed in claim 1, wherein the outer diameter of the central portion in the width direction, of one sheet discharge roller of the pair of sheet discharge rollers, is larger than the outer diameters of the both end portions in the width direction.
 3. The sheet discharging device as claimed in claim 2, wherein the one sheet discharge roller is disposed above the other sheet discharge roller of the pair of sheet discharge rollers.
 4. The sheet discharging device as claimed in claim 2, wherein the elastic layer of the one sheet discharge roller has, in the central portion in the width direction, a cylindrical portion without a change of the outer diameter, a length L of the cylindrical portion is 0 L ⅓ L1 with respect to a length L1 in the width direction of the sheet with a maximum size that allows the sheet to pass between the pair of sheet discharge rollers, and the elastic layer has taper shape from the both ends in the width direction of the cylindrical portion to the both ends in the width direction of the one sheet discharge roller.
 5. The sheet discharging device as claimed in claim 2, wherein the elastic layer of the one sheet discharge roller has a crown shape with a maximum outer diameter in the central portion in the width direction of the sheet.
 6. The sheet discharging device as claimed in claim 2, wherein the elastic layer of the one sheet discharge roller has a hardness higher than that of the elastic layer of the other sheet discharge roller of the pair of sheet discharge rollers.
 7. The sheet discharging device as claimed in claim 1, wherein a tube-like resin film is wound around the outer circumference of at least one of the elastic layers of the pair of sheet discharge rollers disposed on the printed surface side of the sheet.
 8. An image forming apparatus comprising: an image forming portion which forms a toner image; a fixing portion which fixes the toner image on a sheet by heat; and a sheet discharging device which discharges the sheet, having passed through the fixing portion, wherein the sheet discharging device includes: a sheet stacking portion on which sheets are staked; and a pair of sheet discharge rollers which discharges a sheet, on which a toner image is fixed by heat, onto the sheet stacking portion in a U-shaped curve in a width direction perpendicular to the discharging direction of the pair of sheet discharge rollers, wherein each of the pair of sheet discharge rollers includes a rotating shaft extending in the width direction and an elastic layer is provided on the rotating shaft and continuously formed to be longer in the width direction than the width of a sheet with a passable maximum size, and at least one elastic layer of the pair of sheet discharge rollers is formed of a foamed material.
 9. The image forming apparatus as claimed in claim 8, wherein the outer diameter of the central portion in the width direction, of one sheet discharge roller of the pair of sheet discharge rollers, is larger than the outer diameters of the both end portions in the width direction.
 10. The image forming apparatus as claimed in claim 9, wherein the one sheet discharge roller is disposed above the other sheet discharge roller of the pair of sheet discharge rollers.
 11. The image forming apparatus as claimed in claim 9, wherein the elastic layer of the one sheet discharge roller has, in the central portion in the width direction, a cylindrical portion without a change of the outer diameter, a length L of the cylindrical portion is 0 L ⅓ L1 with respect to a length L1 in the width direction of the sheet with a maximum size that allows the sheet to pass between the pair of sheet discharge rollers, and the elastic layer has taper shape from the both ends in the width direction of the cylindrical portion to the both ends in the width direction of the one sheet discharge roller.
 12. The image forming apparatus as claimed in claim 9, wherein the elastic layer of the one sheet discharge roller has a crown shape with a maximum outer diameter in the central portion in the width direction of the sheet.
 13. The image forming apparatus as claimed in claim 9, wherein the elastic layer of the one sheet discharge roller has a hardness higher than that of the elastic layer of the other sheet discharge roller of the pair of sheet discharge rollers.
 14. The image forming apparatus as claimed in claim 8, wherein a tube-like resin film is wound around the outer circumference of at least one of the elastic layers of the pair of sheet discharge rollers disposed on the printed surface side of the sheet. 